Cube ice making machine and method

ABSTRACT

Automatic ice making apparatus for making cube ice, including a plurality of upright evaporators having opposite ice-forming surfaces defining plural cells for molding cube ice, water headers for discharging water downwardly onto the surfaces during the freezing cycle, and a compressor and condenser-receiver circuit including an auxiliary receiver circuit including an auxiliary receiver for supplying liquid refrigerant through an expansion valve to the evaporators during the freezing cycle. A conduit between the condenser-receiver and the auxiliary receiver is valved closed about a minute before the end of the freezing cycle to accumulate an overcharge of liquid refrigerant in the condenser-receiver to be converted to flash gas and transferred to the evaporators at the beginning of the harvesting cycle to aid defrost. Return of suction gas having enough wetness to absorb the heat out of the compressor walls during defrost is achieved, and make-up water to a sump is cooled by returning suction gas during the freezing cycle.

United States Patent [1 1 Morris, Jr.

[ CUBE ICE MAKING MACHINE AND METHOD v [76] lnventor William F. Morris,Jr., Raleigh,

[22] Filed: Nov. 2, 1972 [21] Appl. No.: 303,037

Primary ExaminerWilliam E. Wayner Attorney-Thomas B. Van Poole et al.

[ Oct. 23, 1973 [57] ABSTRACT Automatic ice making apparatus for makingcube ice, including a plurality of upright evaporators having oppositeice-forming surfaces defining plural cells for molding cube ice, waterheaders for discharging water downwardly onto the surfaces during thefreezing cycle, and a compressor and condenser-receiver circuitincluding an auxiliary receiver circuit including an auxiliary receiverfor supplying liquid refrigerant through an expansion valve'to theevaporators'during the freezing cycle. A conduit between thecondenserreceiver and the auxiliary receiver'is valved closed about aminute before the end of the freezing cycle to accumulate an overchargeof liquid refrigerant in the condenser-receiver to be converted to flashgas and transferred to the evaporators at the beginning of the 28Claims, 7 Drawing Figures minnow 2a m SHEET 16? 4 CUBE ICE MAKINGMACHINE AND METHOD BACKGROUND AND OBJECTS OF THE INVENTION The presentinvention relates in general to ice making refrigeration systems, andmore particularly to refrigeration systems for making cube ice whereinthe system is cycled alternately through a freezing phase and aharvesting or defrosting phase.

Automatic ice making apparatus involving reversible cycle refrigerationsystems have gone into wide commercial use. In such systems, ice isproduced, usually in the form of an elongated tube or annular cylinder,during the normal refrigeration or freezing phase of the apparatuswhen-condensed liquid refrigerant is admitted to the evaporator, and theice is discharged from the evaporator during the defrosting orharvesting phase when hot gaseous refrigerant is delivered directly fromthe compressor to the evaporator. Such systems have customarily involvedan evaporator having a refrigerant chamber which contains a large volumeof liquid refrigerant at the conclusion of the freezing cycle. To

accomplish proper defrosting and release of the ice from the evaporatorby hot gaseous refrigerant and avoid an undesirable amount of melting ofthe ice as the hot gaseous refrigerant releases the frost bond betweenthe ice and the evaporator ice-forming surfaces, it has been thoughtthat some means must be provided to rapidly pump down the liquid whichoccupies evaporator at the end of the freezing cycle and transfer thisliquid refrigerant to a cold transfer drum or storage tank at thecommencement of the harvesting cycle to store the refrigerant during theremainder of the harvesting cycle. Also, it has been customary to employa liquid trap to extract from the gas returning to the suction side ofthe compressor whatever liquid phase refrigerant is formed bycondensation of hot gaseous refrigerant in the evaporator during thedefrost cycle, to prevent any of the condensed liquid refrigerant fromreturning to the compressor. In such arrangements, only dry gaseousrefrigerant is returned to the compressor during the harvesting cycle,which has poorer thermal transfer characteristics than wet gaseousrefrigerant and is not capable of as-readily absorbing heat out of thecompressor walls. Obviously the necessity of providing facilities forhandling the liquid refrigerant when it is to' be dumped into a transferdrum for storage during the harvesting cycle, and the associated valvingand plumbing, and the provision of such liquid traps in the suctionline, increasesthe complexity and cost of the equipment as well asrequiring relatively large quantities of refrigerant, and in'certainrespects impairs defrost efficiency. I

Efforts have also been made to achieve effective and rapid harvesting ofice from the evaporator of automatic ice making apparatus by cycling hotgaseous refrigerant to the evaporator without dumping or storing theliquid refrigerant which remains in the evaporator at the conclusion ofthe freezing cycle. In such systems, the hot gaseous refrigerant isintroduced into the refrigerant chamber of the evaporator in such a waythat the hot gaseous refrigerant is placed in effective thermal exchangerelation with the liquid refrigerant throughout the entire height of thebody of liquid to quickly vaporize the liquid refrigerant or warm itsufficiently to release the frost bond holding the ice to theice-forming surfaces of the evaporator. Such refrigeration systems haverequired a number of solenoid valves to effect proper selective controlof intercoupling of the components of the refrigeration system toestablish the various phases of operation forming the complete cycle ofoperation of the system and have not achieved the desired operatingefficiency.

It has been discovered that by providing an additional auxiliaryreceiver connected to the usual water cooled condenser-receiver andclosing the outlet from the condenser-receiver to the auxiliary receivera short period before the conclusion of the freezing cycle, andconnecting the hot gas line with the condenser-receiver and theevaporator sections in such a way that liquid refrigerant backed up inthe condenser-receiver will vaporize as flash gas at the beginning ofthe defrost cycle, considerably improved efficiency of the defrost canbe realized. With such an arrangement, sources of heat for releasing theice from the ice-forming surfaces of the evaporator sections areprovided by the flash gas from the condenser which vaporizes immediatelyupon opening of the hot gas solenoid valve to initiate the defrostcycle, as well as being provided by the hot gaseous refrigerant which isbeing delivered directly from the compressor high side; Also,by;providing a suction accumulator which'meters a proper small amount ofliquid into the refrigerant returning to the compressor during defrost,the desired wetness of the returning suction gas can be achieved toabsorb the heat from the walls or mass of the compressor totransmit itto the evaporator for adding further heat of defrost to the evaporator.

An object of the present invention is the provision of novel ice-makingapparatus having a cycle of operation wherein the evaporator is cycledsuccessivelythrough freezing and thawing phases with a novel mode ofoperation, whereby liquid refrigerant is accummulated in a water cooledcondenser-receiver for a'short period before commencement of theharvesting cycle, to provide liquid refrigerant which will vaporize asflash gas and be fed to the evaporator to assist in defrost.

Another object of the present invention is the provision of novelautomatic ice-making apparatus which is alternately cycled throughfreezing and harvesting phases, to produce cube ice with improvedefficiency.

Other objects, advantages and capabilities. of the present inventionwill become ,apparent from the following detailed description, taken inconjunction with the accompanying drawings illustrating a preferredembodiment of the invention. 1 .BRIEF DESCRIPTION OF THE FIGURESv FIG. 1is a diagrammatic view of automatic icemaking apparatus constructed'inaccordance with the present invention; FIG. 2 is a side elevation viewof the automatic ice making apparatus;

FIG. 3 is a vertical section view through tus, taken along the lines 3-3of FIG. 2;

FIG. 4 is a horizontal section view through the ratus, taken along theline 4-4 of FIG. 3;

FIG. 5 is a fragmentary section view'"through the the apparaappaupperportion of the apparatus, taken along the line FIG. 7 is a fragmentarysection view through the lower portion of one of the evaporatorsections, taken along the line 7-7 of FIG. 6.

DETAILED DESCRIPTION OF A PREFERRED EMBQDIMENT Referring to thedrawings, wherein like reference characters designate correspondingparts throughout the several figures, the basic components of the cubeice-making apparatus of the present invention are supported in anelongated, generally rectangular cabinet frame 10 of welded structuralsteel including a base frame portion 1 1, upright end panels 12 and 13and top frame members 14. The frame 10 also includes an uprightintermediate partition 15 defining a compressor compartment between thepartition 15 and the end panel 12 and an evaporator compartment betweenthe partition 15 and end panel 13. In the compressor compartment regionbetween the partition 15 and end panel 12 is a motor driven compressor16 supported on a shelf 17 spaced above the base frame 11, thecompressor having the usual high pressure discharge and low pressuresuction ports. A high pressure discharge line 18 extends from the highpressure discharge port of the ompressor 16 through a conventional oilseparator 19 and a supply conduit 20 to a Tee fitting 21. One branch ofthe Tee fitting 21 connects the hot gaseous refrigerant from the highside of the compressor through a conduit 22 and a manual discharge valve23 to the inlet of the condenser-receiver 24.

The condenser-receiver 24 is of the usual water cooled type having watersupply and return pipes 25A and 25B and a valved by-passed pipe 25C,connecting the water pipes within the condenser-receiver 24 with theconventional exterior water tower. The outlet port of thecondenser-receiver 24 is connected by liquid line 26 through a sightglass 27 and solenoid valve 28 to the inlet of an auxiliary receiver 29having a capacity of about one-third the liquid capacity of thecondenserreceiver 24. The outlet from the auxiliary receiver 29 isconnected by liquid line 30 through a conventional liquid line dryer 31and'sight glass 32 to the bottom inlet of the conventional coil in aknock-out tank or.

suctionaccumulator 33. The upper outlet of the spiral coil in theknock-out tank 33 is connected through liquid supply line 34 having aliquid solenoid valve 35 therein, to the expansion valve 36 and into theinlet of a liquid distributor 37. The liquid distributor 37 has pluraloutlet conduits 38 forming refrigerant distributor tubes of equal lengthhaving coiled portions to fit the longer tubes into confined spaces,with the distributor tubes extending over the tops of the evaporatorsections 39 of evaporator unit 40 and down along side the back end wallsof the evaporator sections 39 to the lower ends thereof.

Each of the evaporator sections 39, nine of which are provided in theexample shown in the drawings, are each formed in one preferredembodimentwith two ice-forming surfaces 41 and 42 of large area havingvertically and horizontally extending metallic sheet mem bers definingmany ice-forming cells or recesses 43 for producing ice cubes of aboutone inch size. For example, each of the ice-forming surfaces 41 and 42may be made up of a base plate 44, vertical divider partitions 45, andhorizontally extending divider partitions 46 which, like the verticaldividers 45, are welded at their inner ends to the base plate 44 and attheir edges to the vertical dividers 45 and incline outwardly anddownwardly at an angle of about 15 to the horizontal. The base plate 44forming each surface 41, 42 is also sloped outwardly at a slight angleto the vertical, to facilitate migration of water discharged against theupper regions of the surfaces 41 and 42 downwardly in contact with themembers 44, 45 and 46 defining the surfaces of the cells 43 without thewater being thrown outwardly away from contact with these surfaces. Themembers 44, 45 and 46 may be make of 16 gauge tinned copper in onesatisfactory example.

Vertical copper pipes arranged parallel to each other and welded orbrazed to the back surfaces of the base plates 44 in alignment with thejunctures of the vertical dividers 45 with the base plate 44 extendsubstantially the full height of the ice-forming surfaces 41, 42 and areformed, for example, of three-eighths inch copper tubes which are rolledor flattened to approximately a one-quarter inch depth measuredperpendicularly to the surface of the base plate 44. These verticalcopper tubes 47 open into and are joined to a transverse top manifold 48at their upper ends and open into and join a bottom manifold 49 at theirlower ends, both of which may be seven-eighth inch outer diametercoppertubes. Within the bottom manifold 49in coaxial relation is a smallerdiameter liquid supply manifold 50, for example of three-eights inchouter diameter copper tube, having about three-sixteenth inch downwardlyopening holes spaced about four and a half inches on centers along theliquid manifold 50. The liquid manifold 50 is closed at one end and isconnected at the other end to one of the liquid distributor tubes 38.The larger diameter bottom manifold 49 is closed at both ends, one ofthe closed ends being apertured for passage of the smaller diameter ofmanifold pipe 50 therethrough, and is connected by a hole adjacent thelatter end to one of a plurality of hot gas distributor tubes 51extending upwardly along the back end wall of the associated evaporatorsection 39 and across the top thereof to a hot gas header 52. The hotgas header 52 communicates with the other branch of the Tee fitting 21through a hot gas supply line 53 having a hot gas solenoid valve 54 anda strainer 55 therein. The top manifold pipes 48 associated with thetubes 47 of each of the ice-forming surfaces 41 and 42 have one endthereof closed and the other end opens into an elongated suction header56', which returns the vapor phase refrigerant from the evaporatorsections through the coils of a forecooler 57, from which the gas isconveyed through line 58 to a gas inlet opening in the upper region ofthe cylindrical side wall of knock-out tank 33. The knock-out tank 33includes the usual U tube, having an open end near the top of the tankinto which the returning gas is drawn and conveyed through aconventional suction filter 59 and suction return lines 60 to thesuction port of the compressor 16.

During the freezing cycle, water is dischargedv downwardly onto theupper portions of the ice-forming surfaces 41, 42 by a plurality ofspray nozzles or discharge tubes indicated at 61 spaced axially alongeach of a plurality of parallel horizontal water header pipes 62, a pairof which are associated with the upper ends of each of the evaporatorsections 39. The transverse horizontal water header pipes 62 eachconnect to a common water supply pipe or header 63 which extends thelength of the evaporator region of the cabinet frame and descends toconnect to the outlet of a motor driven sump pump 64 located in aninsulated sump or receptacle 65. The level of water in the sump 65 ismonitored by a float valve 66 connected to a make up water supply pipecoupled to the city water supply or another desired water reservoir, thefloat valve when open admitting make up water through water supplyconduits 67 to the forecooler 57 located for example in the upper regionof the compressor compartment, from which a return line 68 descends tothe sump 65. Also, an elongated trough 69 inclines upwardly from thesump 65 along the entire length of the zone occupied by the group ofevaporator sections 39 in underlying relation to the evaporators toreceive water which is discharged from the water header pipes 62downwardly along the ice-forming surfaces 41, 42 which does not freezeduring transit along the ice-forming surfaces and return the water tothe sump. A screen or perforated wall at the lower portion of theconveyor trough 69 permits the water to return to the sump but preventsice from passing into the sump, and a screw conveyor 70 driven by anelectric motor, for example through a chain and sprocket drive, isrotated during the discharge cycle to convey ice cubes or groups of icecubes which are dislodged from the ice-forming surfaces of theevaporator sections during harvesting along the upwardly inclined pathdefined by the trough 69 for discharge through the outlet end 71 thereofto other conveyor means for transmitting the ice to other processingstages.

In the operation of the above described refrigeration system, at thebeginning of the freezing cycle, the electric motor for the sump pump 64is energized through suitable timer circuitry in the electrical panelbox 72 to pump water from the sump 65 through sump pump 64 and watersupply pipe 63 to the transverse water headers 62 and out the dischargenozzles or tubes 61, whereupon the water courses or migrates downwardlyalong the surfaces defining the cubic cells 43 of the iceformingsurfaces 41 and 42 of evaporator sections 39. The water which coursesdown these surfaces and which does not freeze into ice during itspassage down the surfaces 41, 42 discharges from the bottom of theevaporator sections 39 into the trough 69 whereit is returned to thesump 65 for recycling through the water circuit. At the same time thesump'pump 64 is placed in operation at the beginning of the freezingcycle, the hot gas solenoid valve 54 is closed and the liquid solenoidvalve 35 is opened by automatic circuitry in the electric panel box 72.When this occurs, the hot gaseous refrigerant compressed in thecompressor 16 is discharged from the high pressure port thereof throughthe discharge line 18, oil separator 19 and conduits 20 and 22 to thecondenser-receiver 24 where the hot gaseous refrigerant is condensed toliquidphase to form a seal of liquid refrigerantat the condenser outlet.The usual practice is'to' charge the system just enough to seal thecondenser outlet with liquid refrigerant, but not drown the condenser. v

The condensed liquid refrigerant is supplied from the receiver 29through the liquid line 30, knock-out tank 33, liquid line 34 and openliquid solenoid valve 35 to the expansion valve 36 which meterstheliquid refrigerant to the distributor 37 for distribution through thedistributor tubes 38 to the liquid manifolds 50 supplying the liquidrefrigerant to the vertical tubes 47 contacting the base plate 44 of theassociated ice-forming surfaces 41, 42 on opposite sides of each of theevaporator sections 39. The refrigerant at low pressure in the verticaltubes 47 of the evaporator sections 39 extracts heat from theice-forming surfaces 41, 42, freezing the water flowing'downwardly overthese surfaces into ice in the cubic form defined by the surfaces of theiceforming cells or recesses 43'. As the refrigerant extracts heat fromthe ice-forming surfaces and the water, the refrigerant begins to returnto vapor phase and is withdrawn from the evaporator tubes 47 through thetop manifolds 48 to the suction header 56. From there, the cool vaporphase refrigerant courses through the coils in the forecooler 57 inthermal exchange with the make-up or city water being supplied to theforecooler when the float valve 66.is open, and is then conveyed throughthe line 58, knock-out tank 33 and suction filter 59 and return line 60to the suction return port of the compressor 16.

When an appropriate thickness of ice has developed on the evaporatorsurfaces 41, 42 which may be determined by any of several known devices,such as by an automatic timer, or by an ice thickness sensor, or otherconventional means, a harvesting or defrosting cycle is commenced. About60 to 90 seconds before commencement of the defrost cycle, the solenoidvalve 28 in the line between the condenser-receiver 24 and the auxiliaryreceiver 29 is closed to overcharge the condenser during this 60 to 90second period and build up a reservoir of liquid refrigerant in thecondenser for providing flash gas which will be transmitted to theevaporators immediately upon commencement'of the defrost cycle. Thepurpose of the auxiliary receiver 29 is to provide a reservoir of liquidrefrigerant for this last sixty to ninety seconds of the refrigerationcycle when the valve 28 is closed, as otherwise'the liquid refrigerantsupply to the evaporators would be exhausted in about ten secondsfollowing closure of the valve 28. To initiate the actual defrost cycle,the hot gas solenoid valve 54 is opened and the liquid solenoid valve 35is concurrently closed. The conditions thus established, and the heatingeffect of the cooling tower water flowing through the condensertubes'connected to the pipes 25A and 25B, which water is preferably at atempera- IlJL Q Of about to promptly flashes the overcharge of liquidrefrigerant in the condenser to produce a substantial volume of flashgas which is admitted throughlthe'hot gas solenoid valve 54-to the hotgas header- 52 and through the distributor tubes 51 and hot gasmanifolds49 to the evaporator tubes 47 to immediately commence thawingof the frost bond holding the I ice cubes to the surfacesof the cells 43on the evaporator surfaces 41, 42. During the initial phases of theharvesting cycle, the flash gas serves as the primary source of heat fordefrosting, and thereafter. the heat of defrosting is supplied by thehot gaseous refrigerant which continues to circulate through thecompressor from the suction returnv line 60 to the hot gas supply line53 through valve 54 and thence to the hot gas header 52 and distributortubes 51. Improved efficiency of defrost is contributed both by thedevelopment of the flash gas from the overcharge build-up in thecondenser-receiver 24 and by the cycling of wet suction gas throughthecompressor and hot gas supply line 53 to the evaporator so as to absorbsubstantially all of the heat out of the walls or mass of the compressorbody to contribute to the defrosting of the frost bond holding the iceon the evaporator surfaces. The proper wetness of the suction gasreturning to the compressor is achieved by withdrawing the liquid whichcondenses during defrost in the evaporator tubes to the suctionaccumulator or knock-out tank 33 designed to accommodate this amount ofliquid without slugging the compressor, and metering the proper amountof this liquid into the suction line 60 returning to the compressor 16to achieve the desired wetness of the suction gas so that it will mosteffectively absorb the heat from the mass of the compressor casing. Inthis way the heat which has accumulated in the walls of the compressorcasing while the compressor is working during the refrigeration cycle isthen absorbed by thermal exchange into the gaseous refrigerant beingdelivered through the hot gas supply line 53 to the evaporators duringthe defrost cycle to obtain beneficial results from the heat stored inthe compressor casing walls.

Thus, by the above described system, two basic sources of heat are drawnupon during the harvesting cycle to thaw the frost bond holding the iceto the iceforming surfaces: the heat provided by the flash gas formedfrom flashing the overcharge of the liquid refrigerant in the condenserat the beginning of the barvesting cycle; and the heat of the hotgaseous refrigerant being delivered from the compressor through the hotgas solenoid valve 54 and line 53 to the header 52,

the heat of the latter being enhanced by providing re-' turn suction gasto the compressor of proper wetness to absorb substantially all of theheat out of the walls of the compressor casing. In this manner, muchmore efficient defrosting is accomplished, facilitating the dislodgementof the ice cubes from the ice-forming surfaces of the evaporator so thatthe system can again return to the freezing cycle. In practice, the unithas a freezing cycle'of approximately 26 minutes and a defrost cycle ofabout 9 minutes. i

It should be recognized that three conditions are important to obtainsubstantial contribution to defrost from the flashing of the overchargeof liquid refrigerant in the condenser. There must be sufficientimmersion of the water tubes in the overcharge of liquid refrigerantwhich is accumulated in the-condenser during the last 60 to 90 secondsof the freezing cycle; there must be a sufficient amount of overchargeof liquid refrigerant in the condenser at the beginning of theharvesting cycle to produce enough volume of flash gas; and there mustbe a substantial flow of approximately 80 to 85 condenser watercirculated from the exterior cooling tower through the water tubes inthe condenser and serving as the heat source. To provide the propertemperature of condenser water, it is desirable to provide a temperaturecontrol on the cooling tower to provide 80 water, for example by settinga fan control on the cooling tower or louvre controls associated ,withthe cooling tower to maintain the cooling water at the desiredtemperature.

As the frost bond holding the ice cubes to the surfaces of theice-forming cells, 43 on the evaporator surfaces is thawed sufficientlyduring the defrost cycle, the vertically elongated sheet of connectedice cubes which forms on each evaporator surface 41, 42 detachs andfalls gravitationally into the conveyor trough 69 where the sheet breaksinto smaller fragments of plural ice cubes and individual ice cubes uponimpact. To avoid bridging of such fragments as might form if the fallingice sheet struck the confronting evaporator surface, separator rods 73hung from horizontal pivot axes adjacent the upper ends of theevaporator surfaces between confronting pairs of evaporator surfaces anddepending downwardly to the level of the lower ends of the evaporatorsections are provided to maintain the falling sheets in the proper path.

The fragments of plural ice cubes and individual ice cubes which arereceived in the conveyor trough 69 are then conveyed to the outlet end71 by the screw con veyor 70, where they are transmitted either byanother screw conveyor or by other collection or transfer means tofurther processing stages. For example, the ice can be passed to a cubeseparator device which separates the sheet fragments into individualcubes with minimal crushing of the cubes to thereby preserve as much aspossible the cube form of the ice, and the ice can then be delivered tostorage bins or to bag filling devices or other use stations.

What is claimed is:

1. The method of making and discharging ice with a refrigeration systemincluding an evaporator, a condenser and a compressor by alternatelycycling the systemthrough a freezing cycle and a harvesting defrostcycle, comprising the steps of coursing refrigerant from the compressorthrough the condenser to the evaporator for a selected freezing periodduring said freezing cycle to condense the refrigerant to form liquidphase refrigerant at the outlet end of the condenser and deliver it tothe evaporator at a metered rate while concurrently discharging waterover exterior ice-forming surfaces of the evaporator to form icethereon, closing the outlet from the condenser to accumulate a selectedovercharge of liquid refrigerant in said condenser for a short periodbefore the end of the freezing cycle, and terminating delivery of liquidrefrigerant to the evaporator to initiate the defrost cycle andconcurrently connecting the condenser directly to the evaporator torapidly convert said overcharge of liquid refrigerant to flash gas andtransfer the flash gas to the evaporator to supply some of the heat forthawing the frost bond adhering ice to the ice-forming surfaces of theevaporator.

2. The method defined in claim 1, including discharging water downwardlyonto said ice-forming surfaces continuously during the freezing cycle,collecting the water discharged onto said surfaces which does not freezethereon and conveying it in a recirculating .path to redischarge it ontosaid surfaces, and terminating the discharge of water onto saidice-forming surfaces throughout the harvesting cycle.

3. The method defined in clairn.2, including passing make-up water froma supply source in thermal exchange relation with gaseous refrigerantreturning from the evaporator to the compressor to cool the make-upwater and comingle it with the recirculating water.

4. The method defined in claim 1, including. passing make-up water froma supply source in thermal exchange relation with gaseous refrigerantreturning from the evaporator. to the compressor to cool the make-up.water, and comingling it with other water to be discharged onto saidice-forming surfaces of the evapora tor. 1

5. The method defined in claim 1, including circulating water at atemperature in the range of about -85F through'said condenser intothermal exchange relation with the overcharge of liquid refrigerantaccumulated therein to prove a source of heat for rapidly vaporizing theovercharge refrigerant.

6. The method defined in claim 3, including circulating water at atemperature in the range of about 8085F through said condenser intothermal exchange relation with the overcharge of liquid refrigerantaccumulated therein to prove a source of heat for rapidly vaporizing theovercharge refrigerant.

7. In ice making apparatus, a refrigeration system adapted to cyclealternately through a freezing cycle and a harvesting defrost cycle,including an enclosed evaporator chamber having an inlet and an outletand including a generally upright, heat conducting wall defining anexterior ice-forming surface, water discharge means for directing wateronto said ice-forming surface adjacent the top thereof during saidfreezingcycle to gravitate downwardly along the surface; a refrigerantcircuit including a compressor, a condenser, a separate receiver, aliquid line section to the evaporator inlet having an expansion valvetherein, and conduit means for coursing refrigerant from said compressorthrough said condenser and receiver in series circuit relation to saidliquid line section to feed liquid refrigerant to the evaporator chamberduring the freezing cycle; a branch conduit connecting the compressordischarge side and the inlet of the condenser with the evaporator inlet,valves in said branch conduit and in said liquid line section andbetween said condenser and receiver, and control means for closing thevalve between the condenser and receiver a selected short time beforethe end of the freezing cycle to overcharge the condenser with liquidrefrigerant and for concurrently closing the valve in said liquid linesection and opening the valve in said branch conduit at the end of thefreezing cycle whereupon the overcharge of liquid refrigerant in thecondenser converts rapidly to flash gas and is delivered to theevaporator chamber to immediately assist thawing of frost bond adheringice to said ice-forming surface.

8.- Ice making apparatus as defined in claim 7, wherein said condenseris a water cooled condenser, and means for circulating water at atemperature of at least about 80F through said condenser in thermalexchange relation with the overcharge of liquid refrigerant therein. v

9. Ice making apparatus as defined in claim 7, wherein said condenser isa condenser-receiver capable of storing liquid refrigerant therein, andsaid separate receiver is an auxiliary receiver having a liquid storagecapacity about one-third that of the condenser-receiver to provide areservoir of liquid refrigerant therein accumulated prior to saidclosing of the valve between the condenser and receiver to supply theexpansion valve with adequate liquid refrigerant from said valve closinguntil termination of the freezing cycle.

10. Ice making apparatus as defined in claim 8, wherein said condenseris a condenser-receiver capable of storing liquid refrigerant therein,and said separate receiver is an auxiliary receiver having a liquidstorage capacity about one-third that of the condenser-receiver toprovide a reservoir of liquid refrigerant therein accumulated prior tosaid closing of the valve between the condenser and receiver to supplythe expansion valve with adequate liquid refrigerant from said valveclosing until termination of the freezing cycle.

11. Ice making apparatus as defined in claim 7, including a suctionreturn line from said evaporator to said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb heat from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.

12. Ice making apparatus as defined in claim 9, including a suctionreturn from said evaporator to said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb heat from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.

13. Ice making apparatus as defined in claim 10, including a suctionreturn line from said evaporator to said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb heat from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.

' 14. Ice making apparatus as defined in claim 7, including a waterrecirculating system for collecting water discharging from saidice-forming surface below the bottom thereof and recirculating thecollected water to the top of said surface for redischarge thereon, andmeans for supplying make-up water in thermal exchange relation withgaseous refrigerant returning from the evaporator to the compressor andcomingling it with said recirculating water.

15. Ice making apparatus as defined in claim 9, including a waterrecirculating system for collecting water discharging from saidice-forming surface below the bottom thereof and recirculating thecollected water to the top of said surface for redischarge thereon, andmeans for supplying make-up water in thermal exchange relation withgaseous refrigerant returning from the-evaporator to the compressor andcomingling it with said recirculating water.

16. Ice making apparatus as defined in claim 11, including a waterrecirculating system for collecting water discharging from saidice-forming surface below the bottom thereof and recirculating thecollected water to the top of said surface for redischarge thereon, andmeans for supplying make-up water. in thermal exchange relation withgaseous refrigerant returning from the evaporator to the compressor andcomingling it with said recirculating water.

17; Ice making apparatus as defined in claim 7, wherein said ice-formingsurface includes a vertically elongated base wall portion, theevaporator including vertically extending transversely spaced, parallel,metallic tubes joined to said base wall portion and spanning the heightthereof, a horizontal top header pipe connected to each of said tubes atthe upper ends thereof and communicating with the suction side of thecompressor, an outer bottom header pipe connected to the lower ends ofeach of said tubes and communicating with branch conduit, and an innerbottom header pipe coaxially located within said outer bottom headerpipe extending the axial length thereof communicating with said liquidline section downstream of the expansion valve and having pluralorifices therein for feeding refrigerant into said outer bottom headerpipe and into said tubes. I

18. Ice making apparatus as defined in claim 9, wherein said ice-formingsurface includes a vertically elongated base wall portion, theevaporator including vertically extending transversely spaced, parallel,metallic tubes joined to said base wall portion and spanning the heightthereof, a horizontal top header pipe connected to each of said tubes atthe upper ends thereof and communicating with the suction side of thecompressor, an outer bottom header pipe connected to the lower ends ofeach of said tubes and communicating with branch conduit, and an innerbottom header pipe coaxially located within said outer bottom headerpipe extending the axial length thereof communicating with said liquidline section downstream of the expansion valve and having pluralorifices therein for feeding refrigerant into said outer boom headerpipe and into said tubes.

19. In cube ice making apparatus, a refrigeration system adapted tocycle alternately through a freezing cycle and a harvesting defrostcycle, including an enclosed evaporator chamber having an inlet and anoutlet and including a generally upright, heat conducting wall definingan exterior ice-forming surface of large area having a plurality ofrecessed cells shaped to form ice cubes, water discharge means fordirecting water onto said ice-forming surface adjacent the top thereofduring said freezing cycle; a refrigerant circuit including acompressor, a condenser-receiver, an auxiliary receiver, a liquid linesection to the evaporator inlet having an expansion valve therein, andconduit means for coursing refrigerant from said compressor through saidcondenser-receiver and auxiliary receiver in series circuit relation tosaid liquid line section to feed liquid refrigerant to the evaporatorchamber during the freezing cycle; a branch conduit connecting thecompressor discharge side and the inlet of the condenser-receiver withthe evaporator inlet, valves in said branch conduit and in said liquidline section and between said condenser-receiver and auxiliary receiver,and control means for closing the valve between the condenserreceiverand auxiliary receiver a selected short time before the end of thefreezing cycle'to overcharge the condenser-receiver with liquidrefrigerant and for concurrently closing the valve in said liquid linesection and opening the valve in said branch conduit at the end of thefreezing cycle whereupon the overcharge of liquid refrigerant inthecondenser-receiver converts rapidly to flash gas and is delivered tothe evaporator chamber to immediately assist thawing of frost bondadhering ice to said ice-forming surface.

20. Ice making apparatus as defined in claim 19, wherein said condenseris a water cooled condenser, and means for circulating water at atemperature of at least about 80F through said condenser in thermalexchange relation with the overcharge of liquid refrigerant therein.

21. Ice making apparatus as defined in claim 19, wherein saidcondenser-receiver is capable of storing liquid refrigerant therein, andsaid auxiliary receiver has a liquid storage capacity about one-thirdthat of the condenser-receiver to provide a reservoir of liquidrefrigerant therein accumulated prior to said closing of the valvebetween the condenser-receiver and auxiliary receiver to supply theexpansion valve with adeaute liquid refrigerant from said valve closinguntil termination of the freezing cycle.

22. Ice making apparatus as defined in claim 19, including a suctionreturn from said evaporator to said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb heat from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.

23. Ice making apparatus as defined in claim 20, including a suctionreturn line from said evaporator to said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb heat from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.

24. Ice making apparatus as defined in claim 21, including a suctionreturn line from said evaporator to. said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb head from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.

25. Ice making apparatus as defined in claim 19, including a waterrecirculating system for collecting water discharging from saidice-forming surface below the bottom thereof and recirculating thecollected water to the top of said surface for redischarge thereon, andmeans for supplying make-up water in thermal exchange relation withgaseous refrigerant returning from the evaporator to the compressor andcomingling it with said recirculating water.

26. Ice making apparatus as defined in claim 19, wherein saidice-forming surface includes a vertically elongated base wall portion,the evaporator including vertically extendingtransversely spaced,paralle, metallic tubes joined to said base wall portion and spanningthe height thereof, a horizontal top header pipe connected to each ofsaid tubes at the upper ends thereof and communicating with the suctionside of the compressor, an outer bottom header pipe connected to thelower ends of each of said tubes and communicating with branch conduit,and an inner bottom header pipe coaxially located within said outerbottom header pipe extending the axial length thereof communicating withsaid liquid line section downstream of the expansion valve and havingplural orifices therein for feeding refrigerant into said outer bottomheader pipe and into said tubes.

27. Ice making apparatus as defined in claim 21,

wherein said ice-forming surface includes a vertically elongated basewallportion, the evaporator including vertically extending, transverselyspaced, parallel, metallic tubes joined to said base wall portion andspanning the height thereof, a horizontal top header pipe connected toeach of said tubes at the upper ends thereof and communicating with thesuction side of the compressor, an outer bottom header pipe connected tothe lower ends of each of said tubes and communicating with branchconduit, and an inner bottom header pipe coaxially located within saidouter bottom header pipe extending the axial length thereofcommunicating with said liquid line section downstream of the expansionvalve and having plural orifices therein forfeeding refrigerant intosaid outer bottom header pipe and into said tubes. i

28. In cube ice making apparatus, a refrigeration system adapted tocycle alternately through a freezing cycle and a harvesting defrostcycle, including an enclosed evaporator chamber having an inlet and anoutlet and including a generally upright, heat conducting wall definingan exterior ice-forming surface of large area having a plurality ofrecessed cells shaped to form ice cubes, water discharge means fordirecting water onto said cie-forming surface adjacent the top thereofduring said freezing cycle; a refrigerant circuit including acompressor, condenser means for condensing refrigerant, a liquid linesection to the evaporator inlet having an expansion valve therein, andconduit means for coursing refrigerant from said compressor through saidcondenser means to said liquid line section to feed liquid refrigerantto the evaporator chamber during the freezing cycle; a branch conduitconnecting the compressor discharge side and the inlet of the condensermeans with the evaporator inlet, valves in said branch conduit and insaid liquid line section and controlling ing the valve in said liquidline section and opening the valve in said branch conduit at the end ofthe freezing cycle whereupon the overcharge of liqid refrigerant in thecondenser means converts rapidly to flash gas and is delivered to theevaporator chamber to immediately assist thawing of frost bond adheringice to said iceforming surface.

1. The method of making and discharging ice with a refrigeration systemincluding an evaporator, a condenser and a compressor by alternatelycycling the system through a freezing cycle and a harvesting defrostcycle, comprising the steps of coursing refrigerant from the compressorthrough the condenser to the evaporator for a selected freezing periodduring said freezing cycle to condense the refrigerant to form liquidphase refrigerant at the outlet end of the condenser and deliver it tothe evaporator at a metered rate while concurrently discharging waterover exterior ice-forming surfaces of the evaporator to form icethereon, closing the outlet from the condenser to accumulate a selectedovercharge of liquid refrigerant in said condenser for a short periodbefore the end of the freezing cycle, and terminating delivery of liquidrefrigerant to the evaporator to initiate the defrost cycle andconcurrently connecting the condenser directly to the evaporator torapidly convert said overcharge of liquid refrigerant to flash gas andtransfer the flash gas to the evaporator to supply some of the heat forthawing the frost bond adhering ice to the ice-forming surfaces of theevaporator.
 2. The method defined in claim 1, including dischargingwater downwardly onto said ice-forming surfaces continuously during thefreezing cycle, collecting the water discharged onto said surfaCes whichdoes not freeze thereon and conveying it in a recirculating path toredischarge it onto said surfaces, and terminating the discharge ofwater onto said ice-forming surfaces throughout the harvesting cycle. 3.The method defined in claim 2, including passing make-up water from asupply source in thermal exchange relation with gaseous refrigerantreturning from the evaporator to the compressor to cool the make-upwater and comingle it with the recirculating water.
 4. The methoddefined in claim 1, including passing make-up water from a supply sourcein thermal exchange relation with gaseous refrigerant returning from theevaporator to the compressor to cool the make-up water, and cominglingit with other water to be discharged onto said ice-forming surfaces ofthe evaporator.
 5. The method defined in claim 1, including circulatingwater at a temperature in the range of about 80*14 85*F through saidcondenser into thermal exchange relation with the overcharge of liquidrefrigerant accumulated therein to prove a source of heat for rapidlyvaporizing the overcharge refrigerant.
 6. The method defined in claim 3,including circulating water at a temperature in the range of about80*-85*F through said condenser into thermal exchange relation with theovercharge of liquid refrigerant accumulated therein to prove a sourceof heat for rapidly vaporizing the overcharge refrigerant.
 7. In icemaking apparatus, a refrigeration system adapted to cycle alternatelythrough a freezing cycle and a harvesting defrost cycle, including anenclosed evaporator chamber having an inlet and an outlet and includinga generally upright, heat conducting wall defining an exteriorice-forming surface, water discharge means for directing water onto saidice-forming surface adjacent the top thereof during said freezing cycleto gravitate downwardly along the surface; a refrigerant circuitincluding a compressor, a condenser, a separate receiver, a liquid linesection to the evaporator inlet having an expansion valve therein, andconduit means for coursing refrigerant from said compressor through saidcondenser and receiver in series circuit relation to said liquid linesection to feed liquid refrigerant to the evaporator chamber during thefreezing cycle; a branch conduit connecting the compressor dischargeside and the inlet of the condenser with the evaporator inlet, valves insaid branch conduit and in said liquid line section and between saidcondenser and receiver, and control means for closing the valve betweenthe condenser and receiver a selected short time before the end of thefreezing cycle to overcharge the condenser with liquid refrigerant andfor concurrently closing the valve in said liquid line section andopening the valve in said branch conduit at the end of the freezingcycle whereupon the overcharge of liquid refrigerant in the condenserconverts rapidly to flash gas and is delivered to the evaporator chamberto immediately assist thawing of frost bond adhering ice to saidice-forming surface.
 8. Ice making apparatus as defined in claim 7,wherein said condenser is a water cooled condenser, and means forcirculating water at a temperature of at least about 80*F through saidcondenser in thermal exchange relation with the overcharge of liquidrefrigerant therein.
 9. Ice making apparatus as defined in claim 7,wherein said condenser is a condenser-receiver capable of storing liquidrefrigerant therein, and said separate receiver is an auxiliary receiverhaving a liquid storage capacity about one-third that of thecondenser-receiver to provide a reservoir of liquid refrigerant thereinaccumulated prior to said closing of the valve between the condenser andreceiver to supply the expansion valve with adequate liquid refrigerantfrom said valve closing until termination of the freezing cycle.
 10. Icemaking apparatus as defined in claim 8, wherein said condenser is acondenser-receiver capable of storing liquid refrigerant thErein, andsaid separate receiver is an auxiliary receiver having a liquid storagecapacity about one-third that of the condenser-receiver to provide areservoir of liquid refrigerant therein accumulated prior to saidclosing of the valve between the condenser and receiver to supply theexpansion valve with adequate liquid refrigerant from said valve closinguntil termination of the freezing cycle.
 11. Ice making apparatus asdefined in claim 7, including a suction return line from said evaporatorto said compressor having a suction accumulator therein for returninggaseous refrigerant of such wetness to the compressor during the defrostcycle as to absorb heat from the compressor walls causing cooling of thelatter below ambient room temperature and transfer of the absorbed heatto the evaporator to aid in defrosting.
 12. Ice making apparatus asdefined in claim 9, including a suction return line from said evaporatorto said compressor having a suction accumulator therein for returninggaseous refrigerant of such wetness to the compressor during the defrostcycle as to absorb heat from the compressor walls causing cooling of thelatter below ambient room temperature and transfer of the absorbed heatto the evaporator to aid in defrosting.
 13. Ice making apparatus asdefined in claim 10, including a suction return line from saidevaporator to said compressor having a suction accumulator therein forreturning gaseous refrigerant of such wetness to the compressor duringthe defrost cycle as to absorb heat from the compressor walls causingcooling of the latter below ambient room temperature and transfer of theabsorbed heat to the evaporator to aid in defrosting.
 14. Ice makingapparatus as defined in claim 7, including a water recirculating systemfor collecting water discharging from said ice-forming surface below thebottom thereof and recirculating the collected water to the top of saidsurface for redischarge thereon, and means for supplying make-up waterin thermal exchange relation with gaseous refrigerant returning from theevaporator to the compressor and comingling it with said recirculatingwater.
 15. Ice making apparatus as defined in claim 9, including a waterrecirculating system for collecting water discharging from saidice-forming surface below the bottom thereof and recirculating thecollected water to the top of said surface for redischarge thereon, andmeans for supplying make-up water in thermal exchange relation withgaseous refrigerant returning from the evaporator to the compressor andcomingling it with said recirculating water.
 16. Ice making apparatus asdefined in claim 11, including a water recirculating system forcollecting water discharging from said ice-forming surface below thebottom thereof and recirculating the collected water to the top of saidsurface for redischarge thereon, and means for supplying make-up waterin thermal exchange relation with gaseous refrigerant returning from theevaporator to the compressor and comingling it with said recirculatingwater.
 17. Ice making apparatus as defined in claim 7, wherein saidice-forming surface includes a vertically elongated base wall portion,the evaporator including vertically extending transversely spaced,parallel, metallic tubes joined to said base wall portion and spanningthe height thereof, a horizontal top header pipe connected to each ofsaid tubes at the upper ends thereof and communicating with the suctionside of the compressor, an outer bottom header pipe connected to thelower ends of each of said tubes and communicating with branch conduit,and an inner bottom header pipe coaxially located within said outerbottom header pipe extending the axial length thereof communicating withsaid liquid line section downstream of the expansion valve and havingplural orifices therein for feeding refrigerant into said outer bottomheader pipe and into said tubes.
 18. Ice making apparatus as defined inclaim 9, wherein said ice-forming surface includes a verticallyeLongated base wall portion, the evaporator including verticallyextending transversely spaced, parallel, metallic tubes joined to saidbase wall portion and spanning the height thereof, a horizontal topheader pipe connected to each of said tubes at the upper ends thereofand communicating with the suction side of the compressor, an outerbottom header pipe connected to the lower ends of each of said tubes andcommunicating with branch conduit, and an inner bottom header pipecoaxially located within said outer bottom header pipe extending theaxial length thereof communicating with said liquid line sectiondownstream of the expansion valve and having plural orifices therein forfeeding refrigerant into said outer boom header pipe and into saidtubes.
 19. In cube ice making apparatus, a refrigeration system adaptedto cycle alternately through a freezing cycle and a harvesting defrostcycle, including an enclosed evaporator chamber having an inlet and anoutlet and including a generally upright, heat conducting wall definingan exterior ice-forming surface of large area having a plurality ofrecessed cells shaped to form ice cubes, water discharge means fordirecting water onto said ice-forming surface adjacent the top thereofduring said freezing cycle; a refrigerant circuit including acompressor, a condenser-receiver, an auxiliary receiver, a liquid linesection to the evaporator inlet having an expansion valve therein, andconduit means for coursing refrigerant from said compressor through saidcondenser-receiver and auxiliary receiver in series circuit relation tosaid liquid line section to feed liquid refrigerant to the evaporatorchamber during the freezing cycle; a branch conduit connecting thecompressor discharge side and the inlet of the condenser-receiver withthe evaporator inlet, valves in said branch conduit and in said liquidline section and between said condenser-receiver and auxiliary receiver,and control means for closing the valve between the condenser-receiverand auxiliary receiver a selected short time before the end of thefreezing cycle to overcharge the condenser-receiver with liquidrefrigerant and for concurrently closing the valve in said liquid linesection and opening the valve in said branch conduit at the end of thefreezing cycle whereupon the overcharge of liquid refrigerant in thecondenser-receiver converts rapidly to flash gas and is delivered to theevaporator chamber to immediately assist thawing of frost bond adheringice to said ice-forming surface.
 20. Ice making apparatus as defined inclaim 19, wherein said condenser is a water cooled condenser, and meansfor circulating water at a temperature of at least about 80*F throughsaid condenser in thermal exchange relation with the overcharge ofliquid refrigerant therein.
 21. Ice making apparatus as defined in claim19, wherein said condenser-receiver is capable of storing liquidrefrigerant therein, and said auxiliary receiver has a liquid storagecapacity about one-third that of the condenser-receiver to provide areservoir of liquid refrigerant therein accumulated prior to saidclosing of the valve between the condenser-receiver and auxiliaryreceiver to supply the expansion valve with adeaute liquid refrigerantfrom said valve closing until termination of the freezing cycle.
 22. Icemaking apparatus as defined in claim 19, including a suction return fromsaid evaporator to said compressor having a suction accumulator thereinfor returning gaseous refrigerant of such wetness to the compressorduring the defrost cycle as to absorb heat from the compressor wallscausing cooling of the latter below ambient room temperature andtransfer of the absorbed heat to the evaporator to aid in defrosting.23. Ice making apparatus as defined in claim 20, including a suctionreturn line from said evaporator to said compressor having a suctionaccumulator therein for returning gaseous refrigerant of such wetness tothe compressor during the defrost cycle as to absorb heat from thecompressor walls causing cooling of the latter below ambient roomtemperature and transfer of the absorbed heat to the evaporator to aidin defrosting.
 24. Ice making apparatus as defined in claim 21,including a suction return line from said evaporator to said compressorhaving a suction accumulator therein for returning gaseous refrigerantof such wetness to the compressor during the defrost cycle as to absorbhead from the compressor walls causing cooling of the latter belowambient room temperature and transfer of the absorbed heat to theevaporator to aid in defrosting.
 25. Ice making apparatus as defined inclaim 19, including a water recirculating system for collecting waterdischarging from said ice-forming surface below the bottom thereof andrecirculating the collected water to the top of said surface forredischarge thereon, and means for supplying make-up water in thermalexchange relation with gaseous refrigerant returning from the evaporatorto the compressor and comingling it with said recirculating water. 26.Ice making apparatus as defined in claim 19, wherein said ice-formingsurface includes a vertically elongated base wall portion, theevaporator including vertically extending transversely spaced, paralle,metallic tubes joined to said base wall portion and spanning the heightthereof, a horizontal top header pipe connected to each of said tubes atthe upper ends thereof and communicating with the suction side of thecompressor, an outer bottom header pipe connected to the lower ends ofeach of said tubes and communicating with branch conduit, and an innerbottom header pipe coaxially located within said outer bottom headerpipe extending the axial length thereof communicating with said liquidline section downstream of the expansion valve and having pluralorifices therein for feeding refrigerant into said outer bottom headerpipe and into said tubes.
 27. Ice making apparatus as defined in claim21, wherein said ice-forming surface includes a vertically elongatedbase wall portion, the evaporator including vertically extending,transversely spaced, parallel, metallic tubes joined to said base wallportion and spanning the height thereof, a horizontal top header pipeconnected to each of said tubes at the upper ends thereof andcommunicating with the suction side of the compressor, an outer bottomheader pipe connected to the lower ends of each of said tubes andcommunicating with branch conduit, and an inner bottom header pipecoaxially located within said outer bottom header pipe extending theaxial length thereof communicating with said liquid line sectiondownstream of the expansion valve and having plural orifices therein forfeeding refrigerant into said outer bottom header pipe and into saidtubes.
 28. In cube ice making apparatus, a refrigeration system adaptedto cycle alternately through a freezing cycle and a harvesting defrostcycle, including an enclosed evaporator chamber having an inlet and anoutlet and including a generally upright, heat conducting wall definingan exterior ice-forming surface of large area having a plurality ofrecessed cells shaped to form ice cubes, water discharge means fordirecting water onto said cie-forming surface adjacent the top thereofduring said freezing cycle; a refrigerant circuit including acompressor, condenser means for condensing refrigerant, a liquid linesection to the evaporator inlet having an expansion valve therein, andconduit means for coursing refrigerant from said compressor through saidcondenser means to said liquid line section to feed liquid refrigerantto the evaporator chamber during the freezing cycle; a branch conduitconnecting the compressor discharge side and the inlet of the condensermeans with the evaporator inlet, valves in said branch conduit and insaid liquid line section and controlling the outlet from said condensermeans, and control means for closing the valve controlling the outletfrom said condenser means a selected short time before the end of thEfreezing cycle to overcharge the condenser means with liquid refrigerantand for concurrently closing the valve in said liquid line section andopening the valve in said branch conduit at the end of the freezingcycle whereupon the overcharge of liquid refrigerant in the condensermeans converts rapidly to flash gas and is delivered to the evaporatorchamber to immediately assist thawing of frost bond adhering ice to saidice-forming surface.